Teletypewriters allow the hearing or speech impaired to make unassisted telephone calls over the voice grade telephone system. Communicating with teletypewriters through the voice telephone system is called text telephony (“TTY”). There are hundreds of thousands of TTY users in the United States and many more worldwide.
A Telecommunications Relay Service (“TRS”) allows TTY users to communicate with voice telephone users. The TRS employs an operator called a Communication Assistant (“CA”) who serves as an interface between the TTY user and the voice telephone user. The CA voices the TTY messages to the voice telephone user, and types the voice telephone user's spoken responses for the TTY user.
Voice carry over (“VCO”) and hearing carry over (“HCO”) are two additional services offered by the TRS. These services allow the user to combine voice and TTY communication as he deems desirable.
VCO allows a hearing impaired user to communicate using his own voice while receiving typed responses. The CA translates the voice telephone user's spoken responses into text for the VCO user, while the voice telephone user hears the VCO user's spoken voice.
HCO allows a speech impaired person to type his input, while hearing the speech of the voice telephone user. The CA voices the VCO user's typed messages for the voice telephone user, while the HCO user directly hears the voice telephone user's speech.
Many modern TTY devices allow VCO or HCO operation in addition to standard all text operation. Unfortunately, these devices burden the user by requiring the user to manually select the TTY device's mode of operation by, for example, inputting a particular key sequence or toggling a dedicated switch.